This invention relates to apparatus for measuring the water level in a steam generator of the type which compares pressure at a low point in the vessel witht he pressure in a reference leg which is kept full of water extracted from a high point in the steam generator and condensed in a condensation pot at the top of the reference leg. More particularly, it releate to apparatus external to the generator which separates moisture from the steam supplied to the condensation pot.
A commonly used system for measuring water level in a steam generator, such as for example, the steam generator in a pressurized water reactor (PWR) nuclear power generating plant, monitors pressure at a point below the normal level. In order to eliminate the component of the measured pressure generated by the steam in the generator above the water level, the pressure at the low point is compared with the pressure in a reference leg external to the steam generator which is in communication with the steam phase in the vessel. The reference leg is kept full of water by a condensation pot at its upper end which is uninsulated and therefore, condenses steam from the steam generator. Excess condensation runs down against steam flow, through the tubing which connects the condensation pot with an upper tap in the portion of the vessel above the normal fluid level, but below the level of the condensation pot.
This type of water level measurement system has been subject to malfunctions which are attributable to the line between the condensate pot and the upper tap becoming filled with liquid. Indeed, the above described countercurrent flow of steam and liquid is possible only if:
(a) There is no low point in the line between the condensation pot and the upper tap which traps liquid; PA1 (b) the diameter of this line is large enough to avoid sealing by capillary effects; PA1 (c) no excessive liquid content is present with the steam entering the upper tap; and PA1 (d) local effects, mainly dynamic effects due to steam velocity at the upper tap, allow liquid-steam separation.
In existing plants where there has been a malfunction; condition (a) is the first to be verified. Condition (b) is in most cases, marginally met by the 3/4 inch tubing typically used for connection between the condensation pot and the upper tap, however, it is impractical to enlarge the vessel penetration for a larger diameter line. Conditions (c) and (d) cannot be easily monitored, and hence, solutions aimed at satisfying them to prevent the drain line from filling with water have only been based upon assumptions.
Prior efforts have been directed toward preventing excess moisture from entering the line between the upper tap and the condensation pot. At the upper tap location, steam contains many entrained droplets which deposit on the vessels walls, and run down the wall into the tubing, filling it with water. This cause of sealing has been successfully eliminated in some models of steam generators by installing an extension on the tubing which protrudes into the vessel beyond the liquid film at the wall. This solution, however, has proved ineffective for other models of steam generators.
Canadian Pat. No. 1116436 suggests the use of a so called tranquilizer which is a perforated ball which surrounds the top entrance. This device tends to establish static pressure at the tap inlet which in some manner separates liquid from steam. It has been successfully used in France with a few exceptions where the failures have been attributed to excessive local liquid content in the steam.